This invention relates to dielectric heat generators, and more particularly, this invention relates to electrode systems for dielectric heat generators.
Dielectric heat generators are well known in the art and widely used for preheating in the molding of plastics, for quick heating of thermosetting glues in cabinet and furniture making and in a variety of other industrial applications where heating must be introduced uniformly throughout electrically nonconductive materials. In copending patent application Ser. No. 724,600, abandoned, there is described a method and apparatus wherein a dielectric heat generator is employed for heat setting and heat shrinking synthetic-resin yarn and in U.S. Pat. No. 2,433,842 there is described a method wherein a rayon filament is dried by subjecting the filament to radio frequency oscillation.
Conventional dielectric heat generators contain a radio frequency oscillator which produces high frequency sine-wave energy. In some types of dielectric heat generators the material being heated is placed directly between the plates of the capacitor in the tank circuit in the oscillator. In most dielectric heat generators, however, the oscillator is coupled to an external electrode system. The electrode system is usually made up of two electrodes, one coupled through an inductance to the oscillator and the other connected to ground. The physical dimensions of the capacitor which comprises the power electrodes are calculable from the formula: EQU C = 0.224 KA/S;
where C is the electrical capacitance in picofarads needed to resonate with the inductance in the circuit at the operating frequency, K is the dielectric constant of the material between the electrodes, A is the area in square inches of each one of the electrode faces, and S is the spacing distance in inches between the two electrodes.
Theoretically, the electrodes can be as large or long as desired, the only requirement being that the inductance in the tuned circuit be sufficient for resonance of the system.
From a practical standpoint, however, there is an upper limit to the size of the electrodes, which is caused by development of standing waves on the electrode plates. At some fraction of a wavelength, generally recognized as about 1/10-1/12 .lambda., the electrodes begin to behave as an antenna. It is simultaneously resonant in the system as a capacitor, and loadable from the system as a radiating device. At and beyond this point, the voltage and current waveforms, along the "antenna", create hot and cool spots over the length of the electrode, and drying (heating) uniformity is much reduced. Depending on the degree to which the "antenna" is loaded, there is also a power loss due to radiation.